1. Field of the Invention
The present invention relates to a disc-shaped optical data recording medium having a signal recording layer for recording and/or playing information by emitting a light beam thereto, and a transparent protective layer 10 μm to 200 μm thick disposed over the signal recording layer. The invention also relates to a method for manufacturing this optical data recording medium, and to a method for clamping the optical data recording medium.
2. Description of Related Art
Optical discs are known and widely used as a high capacity data storage medium for high density recording and playback of information using a laser beam. These optical discs are broadly categorized as read-only, incrementally writable (multisession), and rewritable. Typical of read-only discs are Compact Discs (CDs) storing audio content and Laser Discs storing video content such as movies. Both incrementally writable (multisession) and rewritable media are widely used in the computer industry, for example, for storing text documents and still image files.
These optical discs typically have a data layer disposed to one main side of a 1.2 mm thick transparent substrate. A protective overcoat is then applied to the data layer, or a protective disc identical to the transparent substrate is bonded by adhesive to the data layer. See, for example, Japanese Laid-Open Patent Publication No. 2001-093193, paragraph [0015] and FIG. 1, and Japanese Laid-open Patent Publication No. 2002-042376, paragraph [0019] and FIG. 1.
Development and introduction of Digital Versatile Discs (DVD), a high capacity optical disc medium, has made it practical for even end-users to record moving picture content (such as movies and video) together with audio to an optical disc. High density media such as DVD have been achieved by using a shorter wavelength laser and an objective lens with a large numeric aperture (NA). However, shortening the beam wavelength and increasing the NA also reduce the tolerance for tilt, the inclination of the disc to the direction of laser beam emission.
Tolerance for tilt can be improved by using a thinner substrate. With DVD media this means, for example, using a 0.6 mm thick substrate assuming a 650 nm laser and 0.60 NA. Because a 0.6 mm thick substrate is mechanically weak and would thus increase tilt, DVD discs have two such substrates bonded together with the data recording surfaces on the inside between the substrates.
By using this laminated structure a transparent reflective layer of gold or silicon, for example, is formed on the data recording surface of one of the two substrates, and a conventional reflective layer of aluminum, for example, is formed on the data recording surface of the other substrate. The substrates are then bonded together with these data recording surfaces facing each other on the inside, resulting in a single-sided, double-layer DVD that can be read from one side of the disc, that is, from the side of the substrate having the transparent reflective layer over the data recording layer. Rewritable DVD media with a similar double-layer construction are also available, but the data recording surface in this case is a rewritable thin-film recording layer instead of a metal mirror layer.
Using a blue-purple laser with an approximately 400 nm wavelength has also been proposed as a way to achieve even higher recording densities. One method uses an approximately 0.1 mm thick transparent protective layer on the read/write side and forms an ultrafine laser spot using an approximately 0.85 NA lens for signal reading and/or writing. The transparent layer can be formed with the following two methods.
(A) Bonding a transparent substrate slightly less than 0.1 mm thick to the signal surface side of a 1.1 mm thick signal substrate using adhesive.
(B) Coating the signal surface side of a 1.1 mm thick signal substrate with an approximately 0.1 mm thick transparent resin layer.
In method (A) a polycarbonate sheet manufactured by casting, for example, is used as the transparent substrate. Thickness variation in such cast sheets is minimal at approximately +/−1 μm. The thickness of the adhesive used to bond this polycarbonate sheet to the signal substrate is also thin and can be easily formed to a uniform thickness. As a result, a transparent protective layer with uniform thickness can be formed on the recording/playback side of the disc.
With method (B) it is difficult to form a uniformly thick coating because of the thickness of the transparent resin, but a low cost, high density optical disc can be achieved because it is not necessary to use sheets manufactured in a high cost casting process.